Broadband systems (including wideband and ultra wideband (UWB)) operate over a broad frequency spectrum which may cause interference to or experience interference from narrow-band systems operating in a portion of the frequency range of the broadband system. For example, ultra-wideband (UWB) is defined as a bandwidth greater than 500 MHz or 20% of the arithmetic center frequency, and the Federal Communications Commission (FCC) designated 3.1-10.6 GHz for unlicensed uses by UWB systems. Due to its wide band characteristics, UWB systems are susceptible to possible interference from other systems utilizing a subset of or partially overlapping frequency spectrum. Most notably for UWB, one such example is the IEEE 802.11a band, which spans from 5.15 to 5.825 GHz.
In order to enable such broadband systems to share a portion of the spectrum with a narrow-band system, band notching techniques have been implemented to notch out the band occupied by the narrow-band system. Traditional band-notching techniques have involved the use of a notch filter (aka band-stop filter or band-rejection filter) to reject unwanted frequency content, designing component filters into the broadband system, and integrating a notch filter into the broadband antenna itself. For planar microstrip monopole antennas in broadband systems, however, one general approach has been to produce notch filter characteristics without the use of traditional notch filters by creating an additional resonance on the antenna at the unwanted frequency to suppress a notch frequency range. Variously shaped structures and their purposeful placement within the radiating sections of the monopole antenna that yields secondary resonance have been implemented, providing a notch filter response over a frequency band of interest. Such structures can take form of slots or additional radiating elements in various shapes within the radiating sections of the antenna. One example band-notching technique for planar monopole antennas is disclosed in the paper “Wideband Planar Monopole Antennas with Dual Band-Notched Characteristics” by Wang-Sang Lee et al (2006, IEEE).
Similar band-notching techniques are known for spiral antennas which are, similar to planar monopole antennas, broadband antennas having a very large frequency spectrum bandwidth for broad band performance. However, some of the advantages of spiral antennas include their ability to be designed to cover the same frequency band as planar monopole antennas while having a comparable or smaller footprint, and generating circularly polarized electromagnetic waves in obi-directional radiation pattern orthogonal to the plane of the antenna. And well-designed spiral antennas can have return loss of less than 2:1 VSWR over broad frequency band. One example band-notching technique for spiral antennas is described in the paper “Archimedean Spiral Antenna with Band-Notched Characteristics” by Dahalan et al (2013, Progress in Electromagnetics Research) where inverted U-shaped slots are embedded in the feed line connecting to one of the two spiral arms.